The invention relates generally to control circuitry for a power supply, in particular reverse current control circuits for a power supply.
A fault tolerant redundant power system comprises a plurality of local power supplies which individually provide their outputs to a common bus. The power signal on the common bus is provided to a device, which appears as an electrical load.
To make the power system fault tolerant, isolation circuitry is often provided at each local power supply which isolates it from the rest of the system in the event of its failure.
Often, the isolation circuitry comprises an OR-ing or blocking diode which connects the output of the local power supply to the common bus. A forward biased blocking diode enables current to flow from the local power supply to the common bus, but blocks reverse current flow therethrough. Accordingly, if the local power supply fails, the blocking action of the diode ensures that the failure does not enable current to flow from the common bus into the circuitry of the failed local power supply.
When a diode is forward biased, there is a significant voltage drop across its junction. The energy lost as current flows through the voltage drop is converted into heat. When output current is in the range of tens to hundreds of amperes (xe2x80x9campsxe2x80x9d), heat generated within a diode can be significant and detrimental to surrounding circuits. Though the use of diodes increase system fault tolerance and reliability, the heat associated with them has a negative effect on system MTBF (mean time between failure).
Other isolation circuitry may utilize an enhanced field-effect transistor (a MOSFET, or FET) and a control circuit instead of a diode. In the most typical case of a positive output voltage and an N-channel FET, the output of the local power supply is connected to the source of the FET; the drain of the FET is connected to the common bus; the gate of the FET is connected to the control circuit. When Vgs, the voltage drop between the gate and source, is lower than the threshold voltage, the FET is OFF and it prevents reverse current flowing into the local power supply. When Vgs is sufficiently higher than the threshold voltage, the FET is ON and will support current flow in either direction.
Traditionally, the control circuit for the FET generated a Vgs having sufficient voltage to cause the FET to operate in saturation during normal power supply operation for significant output current, and Vgs would be dropped below the threshold voltage if a condition that would result in reverse current flow were detected. By driving the FET either into saturation or cut-off, the device is operated like a bipolar ON/OFF switch; operation in the linear region is strictly avoided.
Vsd, the voltage drop between the source and the drain of a FET, may be used to indicate the magnitude and direction of current flowing between the local power supply and the common bus. However, by using a FET which operates only in either its cut-off region or its saturated region, it is difficult to detect and react to a fault condition present between the power supply and the common bus. In a fault condition, the net output current from the power supply typically approaches zero then goes rapidly negative. The saturated ON resistance (Rds-on) of a selected FET chosen to handle high output currents will typically be in milliohms, making the difference between a proper small output current and an improper reverse current in the order of millivolts. It may be difficult to detect the fault condition using current values and voltage values in that range. Further, obtaining a larger voltage signal using a FET having a higher Rds-on or fixed sensing resistor generates more heat under high load conditions.
It is desirable to have isolation circuitry for a power supply which improves upon the characteristics of known blocking systems.
In a first aspect, a control system for selectively isolating a power supply from a common bus is provided. The control system comprises a resistive element providing variable resistance between an input terminal and an output terminal. The variable resistance has one of at least three resistive values. The input terminal is connected to an output path of an output signal of the power supply; the output terminal is connected to the common bus. The resistive element further comprises a control terminal enabling adjustment of the variable resistance. There is also a control element providing a control signal to the control terminal, the control element being responsive to a current flowing between the output path and the common bus. The control element generates the control signal to cause the variable resistance to be set at one of the at least three resistive values to impede flow of the current when the current flows from the common bus to the output path.
The control element may utilize a first voltage signal associated with the input terminal and a second voltage signal associated with the output terminal in providing the control signal. Further, the first voltage signal and the second voltage signal are used to derive a value for current flowing between the common bus and the power supply. Further still, the resistive element may be a FET. The at least three resistive values are resistive values between a source and a drain of the FET and one resistive value of the at least three resistive values is provided when the FET is operating in a linear region. Also, the control element may adjust the control signal to increase the variable resistance, if possible, when there is an increase in the current flowing to the output path from the common bus and when there is a decrease in the current flowing from the output path to the common bus. Also, the control element may adjust the control signal to decrease the variable resistance, if possible, when there is a decrease in the current flowing to the output path from the common bus and when there is an increase in the current flowing from the output path to the common bus. Also, the control element may adjust the control signal to operate the FET in a cut-off region when the current attempts to flow from the common bus to the output path.
The control element may be an operational amplifier. Further, the control signal may be amplified by an intermediary circuit located between the operational amplifier and the control terminal.
Alternatively, the resistive element may be selected from a group comprising a MOSFET, a BJT, a JFET and an IGBT. Further, the control element may be an operational amplifier. The operational amplifier may adjust the control signal to increase the variable resistance as the current flows from the output path to the common bus and as the current decreases. The operational amplifier may adjust the control signal to decrease the variable resistance as the current flows from the output path to the common bus and as the current increases. The operational amplifier may adjust the control signal to operate the resistive element in a cut-off region when the first voltage is smaller than the second voltage.
The control system may further comprise a thermal protection circuit associated with the FET. The thermal protection circuit detects when excessive heat is generated by the FET and then controls the power supply to reduce the excessive heat. Also, the thermal protection circuit may comprise a thermal sensor and a shutdown latch.
Alternatively, the control element may utilize digitized signals to process measurements associated with the current. Also, the control element may be selected from a group comprising a microcontroller, a microprocessor and a controller controlled by a microprocessor.
In a second aspect, a power supply system for use with a common bus providing power to a load is provided. The power supply system comprises a power supply producing an output signal on an output path and a resistive element providing a variable resistance between an input terminal and an output terminal. The variable resistance has one of at least three resistive values. The input terminal is connected to the output path; the output terminal is connected to the common bus. The resistive element further comprises a control terminal enabling setting of the variable resistance. The power supply system also has a control element providing a control signal to the control terminal. The control element is responsive to a current flowing between the output path and the common bus. The control element generates the control signal to cause the variable resistance to be set at one of the at least three resistive values to impede flow of the current when the current flows from the common bus to the output path.
The power supply system may have the resistive element selected from a group comprising a MOSFET, a BJT, a JFET and an IGBT. The power supply system may have the control element as an operational amplifier. The operational amplifier may utilize a first voltage signal associated with the input terminal and a second voltage signal associated with the output terminal in providing the control signal. The control signal may be a function of the first voltage signal and the second voltage signal. The operational amplifier may adjust the control signal to increase the variable resistance as the current flows from the output path to the common bus and as the current decreases. The operational amplifier may adjust the control signal to decrease the variable resistance as the current flows from the output path to the common bus and as the current increases. The operational amplifier may adjust the control signal to operate the resistive element in a cut-off region when the first voltage is smaller than the second voltage.
In a third aspect, a method for selectively isolating a power supply from a common bus is provided. The method comprises determining direction and magnitude of current flowing between an output path of the power supply and the common bus, increasing a resistance value of a resistive element located between the output path and the common bus as the current flows from the output path to the common bus and as the current decreases and decreasing the resistance value as the current flows from the output path and as the current increases. The resistance value is selected from one of the at least three resistive values.
The method may have the resistive element selected from a group comprising a MOSFET, BJT, a JFET and an IGBT with the resistive element operating in a linear region when the current flows from the output path.
In other various aspects, the invention comprise various combinations and subsets of the aspects described above.